Feature door knobs are usually fitted on the centre line of a panelled door. If a letter plate is occupying the middle rail place the knob above it on the muntin, the central vertical member.

Drill a counterbore on the inside of the door to take the head of the screw that is used for fixing the knob, then a clearance hole for the thread that goes through the door (1). The back plate of the knob usually has a locating peg on its back which stops the knob turning when the screw is tightened.

To mark its position press the back plate on to the door so that it leaves an indentation, then drill a shallow recess there for the peg. Fit the assembly and tighten the screw. For a neat finish plug the counterbored hole on the inside to conceal the screw head. 

Fitting a letter plate

Horizontal plates are the more common and are usually fitted in the centre rail of the door. Occasionally, in a fully glazed door, letter plates are placed on the bottom rail, but this makes them awkward to reach. The problem can be overcome by fitting them vertically in one of the door's stiles, but this can weaken a door.
Measure the width of the door and mark its centre. From this mark out the opening which must be only slightly larger than the hinged flap on the letter plate in the middle of the rail on the centre line.
Drill a 12mm clearance hole in each corner of the marked rectangle and cut the opening with a padsaw or power jigsaw, working from the holes. Trim the corners square with a chisel. Mark the centres for the screws, these are usually near the edge so take care when drilling them.